Is it possible to modify the Arduino MEGA 2560 R3 ATMEGA16U2 card to become a joystick HID device? Maybe to use the Joystick library or MMJoy2? Looking to create a 8 axis - 32 button joystick card. Any tutorials out there that are current?
If the Mega has a 16U2 as USB-to-TTL converter, it is possible. There are some articles on the web about 'Arduino Uno as HID'; same should apply to the Mega.
If it's a clone with other USB-to-TTL converter (e.g. CH340), no.
It might be a lot easier to use a board with native USB support. Same form factor would be the Due, but be aware that it's a 3.3V board.
Alternatives are 32U4 based boards (also native USB) and some port expanders (including shift registers) and/or analog multiplexers to get the needed I/O pins.